This invention relates to a process for determining at least one state or condition variable from a model of an RTP system by means of at least one measurement signal measured on the RTP system—the measurement value—which has a dependency upon the state variables to be determined. In particular, this invention relates to a process for determining the temperature of an object, preferably a substrate such as e.g. a semiconductor wafer in a rapid heating unit whereby the object or the substrate is heated by radiation sources.
Rapid heating units for the thermal treatment of substrates such as e.g. semiconductor wafers are widely known in the production of semiconductors. They are used for the thermal treatment of wafers which are preferably made from silicon but, however, can also be made from compound semiconductors such as e.g. II-VI, II-V and IV-IV semiconductors. An important feature with the thermal treatment of semiconductor wafers in a rapid heating unit is accurate control and regulation of the wafer temperature during the thermal treatment. This control and regulation of the wafer temperature requires, once again, accurate determination of the wafer temperature during the thermal treatment in order to control or regulate the rapid heating unit correspondingly. This invention relates especially to the aspect of determining the temperature of a semiconductor wafer during its thermal treatment in a rapid heating unit. In general, the invention relates to determining a state variable from a model of an RTP system which describes the state of the RTP system by means of model parameters.
Different processes for determining the temperature of a semiconductor wafer in a rapid heating unit are known. On the one hand it is known to attach thermal elements to the semiconductor wafers themselves and/or in the direct proximity of the same in order to establish their temperature. The problem arises here, however, that on the one hand, a complex process is required in order to attach the thermal elements to the semiconductor wafers, and on the other hand, they lead to local temperature inhomogeneities because they generally have to remain in thermally conductive contact with the semiconductor wafers, and moreover, effect the radiation field in the fast heating unit, at least near to the thermal element.
Another, contactless process which was made known, for example, in the patent applications and patents DE-A-19852320, U.S. Pat. No. 6,191,392 and U.S. Pat. No. 6,369,363 tracing back to the applicant, uses a contactless temperature measurement. With this contactless temperature measurement a first pyrometer is provided which is directed to one side of the wafer in order to collect radiation coming from the wafer which contains heat radiation from the wafer as well as radiation from the radiation sources reflected on the wafer. Furthermore, a second pyrometer is provided which is directed towards the radiation sources themselves in order to collect radiation coming from the radiation sources which is provided with a modulation. The modulation of the radiation sources is chosen here such that it does not effect the heat radiation of the wafer, but can be measured in the radiation from the radiation sources reflected on the wafer. Using a specific algorithm, it is possible to separate, to a certain extent, the heat radiation of the wafer measured on the first pyrometer from the radiation of the radiation source reflected on the wafer. The temperature of the wafer can then be determined from the heat radiation.
This type of temperature determination requires, however, two pyrometers or detectors, namely a so-called wafer pyrometer (or generally, a detector for measuring the radiation emitted from the wafer and reflected on the wafer or transmitted through the wafer) and a lamp pyrometer (or a second measurement system for collecting the radiation emitted from the lamps or the radiation sources), which are respectively associated with high costs. Furthermore, the lamp pyrometer or the second measuring system not only collects radiation originating from the radiation sources, but also partially radiation originating from the wafer, and this makes it difficult to accurately establish the temperature of the semiconductor wafer and requires additional means for accurately determining the lamp radiation, as described in the patent application DE-A-19852321 tracing back to the applicant. Additional problems arise with a high dynamic of the rapid heating system with regard to the temperature-time behavior of the wafer. If e.g. there are high heating rates of the wafer of over 250° C./sec., the radiation signals of the heat emitters and of the wafer contain frequency portions in the range of the modulation frequency. This results, among other things, in a falsification of the amplitude ratio established in the frequency range from the radiation emitted from the heat emitters and measured by the wafer pyrometer. The transient measurement errors caused by this can have an extremely adverse effect upon the stability and performance of the controlled system with a high dynamic. The contactless temperature determination with the help of modulated radiation sources described above is therefore suitable, preferably for quasi-stationary systems, i.e. for systems or temperature-time processes to which the rapid heating unit wafer system is subjected which are quasi-stationary, i.e. in comparison to the modulation frequency of the radiation sources, they only change slowly with regard to time. A further problem results from the sensor sensitivity and from the requirements with regard to measuring accuracy because the contributions of the modulated radiation are to be very accurately established, because by means of this, an in situ emissivity and/or transmissivity determination of the object (the wafer) takes place.
Starting from the above specified prior art, the task which forms the basis of this invention is to provide a process for determining the temperature of a semiconductor wafer in a rapid heating unit whereby the substrate is heated with a radiation source, which in a simple and cost-effective way makes it possible to reliably determine the temperature of the semiconductor wafer. Furthermore, the task which forms the basis of this invention is to determine a state variable of an RTP system, whereby especially, the state variable can be the temperature of a semiconductor wafer in the rapid heating unit.